The present invention relates to a highly productive radiographic image conversion panel for mammography that can secure a large enough image-forming area to photograph the chest wall and that avoids degradation of characteristics due to moisture absorption, and a method of manufacturing the panel.
There are known a class of phosphors which accumulate a portion of applied radiations (e.g. x-rays, α-rays, β-rays, γ-rays, electron beams, and uv (ultraviolet) radiation) and which, upon stimulation by exciting light such as visible light, give off a burst of light emission in proportion to the accumulated energy. Such phosphors called stimulable phosphors are employed in medical and various other applications.
An exemplary application is a radiographic image information recording and reproducing system which employs a radiographic image conversion panel having a film formed of the stimulable phosphor (stimulable phosphor layer). This radiographic image information recording and reproducing system has already been commercialized as FCR (Fuji Computed Radiography) from Fuji Photo Film Co., Ltd.
In that system, a subject such as a human body is irradiated with x-rays or the like to record radiographic image information about the subject on the radiographic image conversion panel (more specifically, the stimulable phosphor layer). After the radiographic image information is thus recorded, the radiographic image conversion panel is scanned two-dimensionally with exciting light such as laser light to produce stimulated emission which, in turn, is read photoelectrically to yield an image signal. Then, an image reproduced on the basis of the read image signal is output as the radiographic image of the subject, typically to a display device such as CRT or on a recording material such as a photographic material.
The radiographic image conversion panel is typically produced by the steps of first preparing a coating solution having particles of a stimulable phosphor dispersed in a solvent containing a binder, etc., applying the coating solution to a support in panel form that is made of glass or resin, and drying the applied coating.
Phosphor panels are also known that are made by forming a stimulable phosphor layer (hereinafter also referred to as a phosphor layer) on a support through methods of vacuum film deposition (vapor-phase film formation) such as vacuum evaporation or sputtering. The phosphor layer prepared by the vacuum film deposition has excellent characteristics. First, it contains less impurities since it is formed under vacuum; further, it is substantially free of any substances other than the stimulable phosphor, as exemplified by the binder, so it has high uniformity in performance and still assures very high luminous efficiency.
One factor for deterioration of characteristics of the radiographic image conversion panel is moisture absorption by the stimulable phosphor layer.
The stimulable phosphor layer, in particular, the alkali halide-based stimulable phosphor layer having favorable characteristics, has high moisture absorption and easily absorbs moisture even in a normal environment (normal temperature/normal humidity). As a result, deterioration of sharpness of a reproduced image or the like occurs due to deterioration of photostimulated luminescence characteristics, that is, sensitivity, or deterioration of crystallinity of the stimulable phosphor (destruction of columnar crystals in the case of the alkali halide-based stimulable phosphor having a columnar structure, for example).
In order to solve such problems, the phosphor layer of the radiographic image conversion panel is sealed with a moisture-proof member (see JP 2003-248093 A, JP 2003-149396 A and JP 2002-71899 A).
JP 2003-248093 A discloses a radiographic image conversion panel including a support, a stimulable phosphor layer formed on the support, and a protective layer. In this radiographic image conversion panel, when the distance between an end face of the radiographic image conversion panel and an end face of the stimulable phosphor layer formed on the support is represented by t, t is 4 mm or less on each of the four sides of the radiographic image conversion panel.
Also disclosed in JP 2003-248093 A is a radiographic image conversion panel 100 in which, as shown in FIGS. 9A and 9B, a polyethylene terephthalate sheet or other plastic sheet used for a protective film 106 is adhered to a rear face 102a of a glass support 102 on which a phosphor layer 104 is formed, thereby sealing the phosphor layer 104 therein. In the radiographic image conversion panel 100, a polyethylene terephthalate sheet slightly larger than the support 102 is superposed on the support 102 and the polyethylene terephthalate sheet and the support 102 are fully bonded together. Then, three of the four sides of the polyethylene terephthalate sheet are firmly bonded to the rear face 102a of the support 102 using a sealant or by heat bonding. Thereafter, the pressure is reduced through the remaining side to bring the protective film 106 into intimate contact with the phosphor layer 104 and the phosphor layer 104 is completely sealed therein using a sealant or by heat bonding.
JP 2003-248093 A also discloses a radiographic image conversion panel 110 which has, as shown in FIGS. 10A and 10B, a spacer 116 made of glass on each of the four sides of the panel. On one of the four sides, the spacer 116 is bonded to a support 112 and to a protective layer 118 made of glass to seal a phosphor layer 114 at a position at which the distance from an end face of the phosphor layer 114 to an end face of the panel is equal to the width of the spacer 116.
In the radiographic image conversion panel 110, on one of the four sides, the distance S between an end face of the phosphor layer 114 and an end face of the radiographic image conversion panel 110 is no larger than the width of the spacer 116. If, for example, the width S of the spacer 116 is 1 mm, the photograph area starts immediately from the edge of the radiographic image conversion panel 110 on this side.
JP 2003-149396 A discloses a phosphor screen and a phosphor panel which have a support, a phosphor layer formed on the support, and a moisture-proof protective over-coating layer. The phosphor layer is smaller in surface area than the support, so that the edges of the phosphor layer do not reach the edges of the support. The over-coating layer is formed on the support to cover the phosphor layer.
JP 2002-71899 A discloses a radiographic image conversion panel in which moisture-proof protective films are used to cover the top face and bottom face of a phosphor sheet having a stimulable phosphor layer formed on a support, and which uses an impulse roller to heat seal the perimeter of the phosphor sheet.
The radiographic image conversion panels 100 and 110 disclosed in JP 2003-248093 A and shown in FIGS. 9A and 9B, and FIGS. 10A and 10B both have a problem in that their manufacture processes are complicated, which leads to low productivity and high manufacture cost.
The manufacture process of the radiographic image conversion panel 100 shown in FIGS. 9A and 9B is particularly complicated, because a polyethylene terephthalate sheet has to be bonded to the rear face 102a on three sides with a sealant, by heat bonding, or the like.
In addition, the radiographic image conversion panel 110 shown in FIGS. 10A and 10B cannot obtain enough sealing effect when the width S of the spacer 116 is small, because the sealing portion is narrowed and the joining area between the spacer 116 and the protective layer 118 becomes small.
The phosphor screen and phosphor panel of JP 2003-149396 A have a problem in that no image can be formed on the perimeter of the phosphor screen or the phosphor panel, because the phosphor layer is smaller in surface area than the support. The screen or panel thus fails to secure a large enough image-forming area to photograph the chest wall required for mammography.
Further, an image cannot be formed on the perimeter of the radiographic image conversion panel of JP 2002-71899 A, in which the perimeter is heat sealed and therefore a margin for heat seal is needed. The screen or panel thus fails to secure a large enough image-forming area to photograph the chest wall required for mammography.